1. Field of the Invention
The present invention relates to a projector using a liquid crystal display device. And also, the present invention relates to a liquid crystal projection TV incorporating the liquid crystal projector.
2. Description of the Related Art
In recent years, a technique for manufacturing a semiconductor device in which a semiconductor thin film is formed on an inexpensive glass substrate, such as a thin film transistor (TFT), has been rapidly developed. The reason is that the demand for an active matrix type liquid crystal display device (liquid crystal panel) has been increased.
The active matrix type liquid crystal panel is structured such that a TFT is disposed for each of several tens to several millions pixel regions disposed in matrix, and an electric charge going in and out of each pixel electrode is controlled by the switching function of the TFT.
Above all, a projection type display device using an active matrix type liquid crystal panel, a so-called projector has rapidly broadened the market. The reason is that the liquid crystal projector is superior in color reproducibility, is small, is lightweight, and has low power consumption, as compared with a projector using a CRT.
The liquid crystal projector is classified into a three-panel type and a single panel type by the number of active matrix type liquid crystal panels to be used.
FIG. 16 shows an example of a three-plate type liquid crystal projector. Reference numeral 1601 denotes a light source, 1602 and 1603 denote dichroic mirrors for selectively reflecting light in a wavelength region of R (red) and G (green), respectively. Reference numerals 1604, 1605, and 1606 denote total reflection mirrors, and 1607, 1608, and 1609 denote transmission type liquid crystal panels corresponding to R, G, and B. Reference numeral 1610 denotes a dichroic prism, and 1611 denotes a projection lens.
In the three-plate type liquid crystal projector, pictures corresponding to the three primary colors of red, green, and blue are displayed on the three black and white display liquid crystal panels 1607, 1608, and 1609, and the liquid crystal panels are illuminated with beams of light of the three primary colors corresponding to the pictures. The obtained pictures of the respective primary color components are synthesized by the dichroic prism 1610 and are projected on a screen. Thus, the three-plate type liquid crystal projector is superior in display properties (resolution, screen illumination, color purity). However, since the liquid crystal panels and optical parts (lenses, mirrors, and the like) for three systems are required, the optical system becomes complicated and miniaturization is difficult. Moreover, since the expensive dichroic prism is required, the cost becomes very high.
On the other hand, in the single plate type liquid crystal projector, by the same system as a conventional direct view type liquid crystal display device using a color filter, the obtained color picture is projected on a screen by a method of driving each of the R, G, and B pixels. FIG. 17 is a structural view showing a conventional single plate type projector. Reference numeral 1701 denotes a light source, 1702 denotes a condensing lens, 1703 denotes a liquid crystal panel, 1704 denotes a projection lens, and 1705 denotes a screen.
Since the number of optical parts of the single plate type liquid crystal projector is merely 1/3 of those of the foregoing three-plate type liquid crystal projector, the single type liquid crystal projector is superior in cost, size, and the like. However, in the case where the same liquid crystal panel is used for both the three-plate type and the conventional single plate type, while three colors are overlapped on one pixel in the three-plate type, one pixel can be used only as one color pixel in the single plate type, so that the picture quality of the single plate type is inferior to that of the three-plate type. Moreover, in the above single plate type liquid crystal projector, a desired color picture is obtained by making an unnecessary component absorbed by a color filter. Thus, only 1/3 of the white light incident on the liquid crystal panel transmits, so that use efficiency of light is poor.
Although a method of making a light source brighter has been adopted to improve the brightness of the above single plate type liquid crystal projector, there have occurred problems with respect to the heat generation due to light absorption of a color filter and its light resistance.
Then, for the purpose of overcoming the defects of the conventional single plate type liquid crystal projector, a liquid crystal projector using three dichroic mirrors and a microlens array has been devised.
Reference will be made to FIG. 18. FIG. 18 is a structural view showing an optical system of the above single plate type liquid crystal projector. Reference numeral 1801 denotes a white light source including a lamp and a reflector. Reference numerals 1802, 1803, and 1804 denote dichroic mirrors which reflect light in the wavelength regions of blue, red, or green, respectively. Reference numeral 1805 denotes a microlens array which is constituted by a plurality of microlenses. Reference numeral 1806 denotes a liquid crystal panel, which makes display in a TN (twisted nematic) mode. Incidentally, the liquid crystal panel 1806 operates in a normally white mode in which white display is made when a voltage is not applied. Reference numeral 1807 denotes a field lens, 1808 denotes a projection lens, and 1809 denotes a screen.
The light 1801 source emits the white light having a spectrum of red, green, and blue. The light source 1801 is set so that the parallelity of the emitted white light becomes high. The reflector is used to effectively use the white light emitted from the lamp.
The white light emitted from the light source 1801 is incident on the dichroic mirrors 1802, 1803, and 1804. These three dichroic mirrors are disposed at different angles so that the white light from the light source 1801 is separated into beams of light of three primary colors (red, green, and blue), and these three light beams are incident on the microlens array 1805 at different angles.
The dichroic mirror 1802 reflects only the ray of light in the blue wavelength region and transmits other beams of light. The dichroic mirror 1803 reflects only the ray of light in the red (R) wavelength region among the beams of light having passed through the dichroic mirror 1802, and transmits other beams of light. The dichroic mirror 1804 reflects the ray of light in the green wavelength region among the beams of light having passed through the dichroic mirrors 1802 and 1803. By adopting such a structure, it is possible to separate the white light emitted from the light source 1801 into three colors.
Reference will be made to FIG. 19. As shown in FIG. 19, one microlens corresponds to three pixels of the liquid crystal panel 1806 corresponding to the three primary colors of R, G, and B.
The microlens array 1805 distributes the above separated beams of light of the three primary colors to the corresponding pixels and condenses the beams.
Like this, in the single plate type liquid crystal projector having the above structure, after the white light is separated into beams of light of the three primary colors of R, G, and B, the respective beams of light are made incident on an opening portion of the corresponding pixel of the liquid crystal panel by the microlens, so that the light can be used at efficiency not less than three times the foregoing single plate type liquid crystal projector using the color filter.
However, in the liquid crystal projector using this microlens, the light flux condensed to the respective pixels by the microlens diverges in a large angle range after it has passed through the liquid crystal panel. Thus, unless a projection lens with a large aperture is used, the light flux can not be completely used, so that the screen illumination is lowered.
In the liquid crystal panel used in the liquid crystal projector using this microlens, a TN (twisted nematic) mode is adopted. Moreover, a normally white mode in which a white state is made when a voltage is not applied to a liquid crystal, is used. In the TN mode, although a higher contrast is obtained in the case where display at the liquid crystal panel is made in the normally white mode than the normally black mode (mode in which a black state is made when a voltage is not applied to a liquid crystal), if defects occur in a pixel portion TFT of the liquid crystal panel, they are apt to become bright point defects. In the case where a picture is enlarged and is displayed as in the projector, especially the foregoing pixel defects have a bad influence on the picture display.